1. Field of the Invention
This invention relates generally to gas discharge lamps, and more specifically to miniature or small-diameter gas discharge lamps and methods of manufacture.
2. Description of the Related Art
One type of gas discharge lamp is the traditional fluorescent tube lamp. These lamps are made by coating an inner surface of a glass tube with phosphor material, sealing a gas mixture (e.g., mercury vapor, neon, and argon) within the glass tube, and installing electrodes at the ends of the glass tube. The lamp is operated by applying sufficient electrical power to the electrodes (either AC or DC) to ionize the internal gas mixture of the lamp. Electrons traveling between the electrodes strike mercury atoms which react by generating ultraviolet light. This ultraviolet light strikes the phosphor material within the glass tube, and the phosphor material generates visible light in response. Other types of gas discharge lamps (e.g., neon lamps or ultraviolet sterilizing lamps) do not have an inner coating of phosphor material, and the gas sealed within the tube is selected to provide the desired wavelengths of light. Such lamps are typically formed in a batch process in which lamp tubes are bent, welded, or cut to shape and length, then coated with phosphor, fitted with electrodes, and then vacuum processed.
Fluorescent lamps with phosphor material inside the tube suffer from shortened life spans and reduced light generation due to various effects which degrade the phosphor material. For example, the phosphor material is damaged by heat from the arc stream, by exposure to mercury vapor which bonds to the phosphor material, and by sputtered materials from the electrodes depositing onto the phosphor material. In addition, in traditional lamp manufacturing, residual materials are removed from the phosphor suspension deposited onto the inner surface of the tube so that these residual materials do not outgas and contaminate the atmosphere inside the finished lamp. This removal process, termed “lehring,” involves heating the coated tube and flushing it with air to burn out the residual suspension materials, and this heating can cause some degradation of the phosphor material. The glass tube is then filled with the desired gaseous atmosphere. This high-temperature lehring process contributes to the degradation of the phosphor material.
Furthermore, conventional gas discharge lamps utilize internal metallic electrodes with external electrical connections which are bonded to the glass tube by hermetic glass-to-metal seals at the tube ends. These glass-to-metal seals avoid leakage or contamination (e.g., by water vapor) of the gaseous atmosphere within the glass tube. They are formed by a process which includes vacuum baking the assembly to a final seal. This vacuum baking process to form the seals also contributes to the degradation of the phosphor material. Failure of these glass-to-metal seals also limits the lifetime of the gas discharge lamp.
It is difficult to miniaturize fluorescent lamps. As the diameters of fluorescent lamps are reduced, it becomes more and more difficult to employ conventional methods of manufacture. The small diameter of the tube creates difficulties in applying the phosphor material to the inner surface of the tube and in lehring and in vacuum baking the residual materials away, thereby limiting the length of the tube of the miniature fluorescent lamp. Existing procedures for applying the internal phosphor coating by flushing solvent-based or water-based phosphor suspensions through reduced-diameter tubes can produce inhomgeneities in the internal phosphor coating. In addition, conventional methods for forming electrodes and glass-to-metal seals present difficulties as the diameter of the gas discharge lamp is reduced.